H5N1 is a looming threat to human health. A safe and effective vaccine is the best way to prevent large-scale outbreaks in the human population. Parainfluenza virus 5 (PIV5), a paramyxovirus, causes kennel cough in dogs, but is not known to cause any illness in humans. PIV5 can be produced in high titers (up to 8x108 plaque forming units (PFU)/ml) in many cells including Vero cells, which are WHO approved for vaccine production. PIV5 can infect human cell lines as well as primary human cells. In our preliminary studies, a single dosage of 104 PFU of a live recombinant PIV5 expressing a HA gene from the H3 subtype provided immunity against influenza A virus subtype 3 infection in mice. We hypothesize that PIV5 is a good vector for vaccine development and we propose to test PIV5 as a vector for H5N1 vaccine. We will focus our efforts on following specific aims: 1. Testing recombinant PIV5 expressing HA of H5N1 (PIV5-H5) as a vaccine for H5N1; 2. Testing the potential of other H5N1 proteins expressed using PIV5 as antigens for vaccine; and 3. Testing mutant PIV5 viruses as vaccine vectors. PUBLIC HEALTH RELEVANCE: Influenza A virus causes significant morbidity and mortality each year. Strains currently circulating in humans (i.e. H1N1, H1N2, and H3N2) infect up to 15% of the world population and cause an average of 36,000 deaths and 226,000 hospitalizations in the United States (28), as well as millions deaths world wide. Sporadic outbreaks of pandemic influenza have caused significant mortality over the past century, most notably the Spanish flu of 1918, and have caused over 50 million deaths world wide. On the horizon is another potentially pandemic strain of influenza, H5N1. This avian influenza virus has most notably emerged in Southeast Asia and resulted in the destruction of millions of birds, infected 309 people, caused 187 human fatalities since 2003 (WHO, Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO, 31 May 2007), and threatens to become the next pandemic. Currently, the only FDA-approved vaccine against H5N1 has serious limitations, particularly as it has to be given twice and requires substantial higher concentrations of the vaccine to achieve a moderate level of efficacy compared to conventional influenza vaccines. Conventional vaccines utilizing the HA and NA of H5N1 viruses have been poorly immunogenic and have safety and production issues. A live-attenuated H5N1 vaccine has been generated by reverse genetics, but the risk of generating a reassortant prohibits use of this vaccine in most instances. Inactivated virus vaccines have also been derived by reverse genetics and produced in large quantities, but preliminary results from NIAID clinical trials suggest that efficacy will require both multiple immunizations and 6 times the standard influenza virus antigen dose, i.e. 90 ug instead of 15 ug of antigen while only providing protection in a subset (~50%) of vaccinated individuals. Nonetheless, FDA has recently approved the inactivated H5N1 vaccine for use in people between age 18 and 64, an age group that is not the most vulnerable to influenza virus infection. Thus, there is a rationale need for new vaccine strategies that provide increased immunogenicity and safety. Parainfluenza virus 5 (PIV5), a paramyxovirus, causes kennel cough in dogs, but is not known to cause any illness in humans. PIV5 can be produced in high titers (up to 8x108 plaque forming units (PFU)/ml) in many cells including Vero cells, which are WHO-approved for vaccine production. PIV5 can infect human cell lines as well as primary human cells. In our preliminary studies, a single dosage of 104 PFU of a live recombinant PIV5 expressing a HA gene from the H3 subtype provided immunity against influenza A virus subtype 3 infection in mice. In this proposal, we hypothesize that PIV5 is a good vector for vaccine development and we propose to test PIV5 as a vector for H5N1 vaccine.